1. Field of the Invention
The present invention relates generally to manholes and to the construction, replacement and alteration of such structures in the field. More particularly, the invention relates to a composite manhole system comprising a corrosion resistant plastic cylinder surrounded by a rigid concrete frame which provides structural integrity but does not entirely encapsulate the plastic cylinder, thus facilitating manipulation in the field.
2. Discussion of the Prior Art
Manhole structures provide access to underground facilities, such as sewer systems and pipelines, for the purposes of repair, cleaning, maintenance and inspection. For convenience, manholes are usually placed at frequent intervals along a sewer line. In addition, manholes often provide a junction point for two or more intersecting pipelines.
There are numerous problems associated with present manhole technology, many of which relate to the harshness of the environment. Manholes are constantly exposed to eroding, oxidizing and corrosive elements associated with the soil on the outside and with the acidic sewage that flows on the inside. The concrete frequently used to construct manholes provides the strength necessary to withstand some of the harshness of the environment and load concentration.
The inflow of rainwater into sewer systems causes overloading of sewer plants, resulting in increased expenses associated with the treatment of larger flow volumes. Studies have shown that inflow and infiltrating rainwater can increase the flow in a sewer system up to 40% and that up to 75% of such inflow may result from defects in manholes. Exfiltration from manholes allows seepage of raw sewage into the surrounding soil and eventually into rivers and streams. This defeats the whole purpose of a sewer system, which is to isolate raw sewage away from the rest of the water supply until it can be treated and detoxified.
Another drawback to common manhole technology relates to the difficulty of construction and assembly. Manhole structures are commonly built using brick, tile or concrete, making it necessary to build or cast the structures in place in the field. This adds extra labor costs which could be greatly reduced using prefabricated manhole systems. However, even precast concrete manholes are problematic because they are heavy and difficult to manipulate. Also, concrete manholes are difficult to cut, decreasing their versatility in situations where it becomes necessary to connect new inlet and outlet pipes. Moreover, concrete manholes, whether they are precast or constructed in the field, are still highly subject to corrosive forces and joint leaks at the connections.
The problems associated with conventional concrete manhole technology have led workers in the field to utilize prefabricated manhole structures made of reinforced plastic. For example, U.S. Pat. Nos. 3,715,958 and 3,938,285 both teach large, preformed manhole structures made entirely of plastic and glass fiber. These structures effectively overcome the problems of corrosion associated with conventional concrete manhole technology but they are bulky, difficult to handle, expensive to ship and lack the versatility required for proper sizing and customization to a particular site. Furthermore, they lack structural integrity and resistance to crushing by external forces.
To add strength and versatility to preformed plastic manholes, improvements such as stackable segments and plastic ribs or stiffeners have been developed. For example, U.S. Pat. No. 3,974,599 teaches an underground vault or manhole featuring increased rigidity of the body by the addition of reinforced plastic mortar rings and struts. In addition, U.S. Pat. No. 4,089,139 teaches stackable manhole units with internal reinforcing ribs which are an integral part of the body of each unit.
U.S. Pat. No. 5,386,669 further improves upon manhole technology by disclosing a complex modular manhole system comprising stackable double-walled units. The double walls give two potential sources of strength. First, the cavity between the walls may be filled with a structural material such as concrete that can be pumped through a hole drilled in the upper portion of each unit. Second, adjacent recesses are placed in the outer walls of some units to form structural ribs between the recesses. One disadvantage of this type of system is that the use of rebar to reinforce the concrete poured into the units is difficult and impractical. A second disadvantage is that, once the concrete is poured into the units, the manhole is entirely encapsulated by a thick, hard shell, making it difficult to later cut through the structure when it becomes necessary to splice in new pipelines.
Other patents have also disclosed manhole technology which takes advantage of the strength of concrete and the corrosion resistance of plastic. U.S. Pat. No. 4,540,310 discloses a manhole system comprising a stackable concrete lower section and an upper plastic cylindrical sleeve which has a integral flange extending outwardly from the bottom. The bottom of the sleeve including the flange is integrally cast into the top concrete riser section of the lower concrete structure. When the whole structure is placed underground, the top plastic sleeve is surrounded by the sleeve portion of an iron frame and grade rings of a desired thickness, the lower most of which contacts the flange portion of the iron frame. The manhole cover rests on the top of the sleeve portion of the iron frame. The grade rings are required to bring the manhole cover up to grade level. This sleeve is advantageous because it can be made extra long, preincorporated into the concrete riser section, and then cut to the desired height at the jobsite. One disadvantage of the system is that the bottom section is made entirely of concrete and is subject to the typical corrosive forces and joint leaking problems discussed above. Another disadvantage is that the plastic is only used for the top sleeve portion and is fully encapsulated by the surrounding iron frame. Although this frame provides necessary structural reinforcement, it does not allow workers easy access when it becomes necessary to splice a new pipeline into the plastic sleeve.
U.S. Pat. No. 5,383,311 discloses a manhole system wherein concrete is cast against a plastic preshaped liner containing integral, hollow, outward projections. Once the concrete is cast, the hollow projections become filled with concrete and protrude into the resulting outer layer of concrete, forming a tight lock between the plastic liner and the concrete. This system is disadvantageous because the liner is entirely encapsulated by concrete, making the unit extremely heavy and difficult to manipulate. In addition the encapsulation by concrete makes it extremely difficult to cut as necessary to add pipe connections.
As seen above, the prior manhole technology is not sufficiently versatile and suffers from various disadvantages and problems. For example, prior art that employs concrete by itself allows for corrosion and deterioration over time. On the other hand, plastic alone does not provide the strength that concrete does. The prior art that discloses the use of both plastic and concrete does not allow for easy access to the plastic for the purpose of splicing in new pipelines. Moreover, the technology disclosed by the prior art is highly labor and cost intensive, owing either to the amount of construction which must be done at the jobsite or to the difficulty of manipulating the bulky components of such manhole systems.